A ferroelectric memory is receiving attention associated with progress of portable terminals in recent years, such as a portable telephone, a notebook computer, a personal digital assistant (PDA) and the like. This is because a ferroelectric memory is advantageous particularly for a multimedia equipment owing to a high writing speed and realization of a large capacity, and no electric power is necessary for maintaining data to realize low electric power consumption.
A ferroelectric memory utilizes polarization characteristics of a ferroelectric material. The direction of polarization is arbitrarily controlled with an external electric field to maintain binary data, i.e., “0” and “1”, and the data can be maintained even upon turning off power.
However, only products having a small capacity of from 4 to 256 kbit are commercialized. It is the current situation that application of a ferroelectric memory to a large capacity product in Mbit level is hindered by the problem of the ferroelectric material itself.
The lamellar structure ferroelectric material (BiAm−1BmO3m+3) such as plumbum zirconate titanate (PZT; PbZrxTi1−xO3) perovskite ferroelectric material (ABO3), strontium bismuth tantalate (SBT; SrBi2Ta2O9) and bismuth titanate (BIT; Bi4Ti3O12) highlighted under donation of La, which are currently used, are necessarily baked at a high temperature of about from 600 to 800° C. for a long period of time (T. Nakamura, Technical Research Report of Institute of Electronics, Information and Communication Engineers, ED97-208 (1998), p. 25–32; T. Eshita, et al., Technical Research Report of Institute of Electronics, Information and Communication Engineers, ED98-242 (1999), p. 21–26; and M. Yamaguchi, “Studies on Formation and Evaluation of Bismuth Titanate Thin Film on Silicon Substrate”, Academic Dissertation of Shibaura Institute of Technology (1998), p. 39–47). Such crystallization at a high temperature for a long period of time is necessary not only for derives sufficient characteristics of the ferroelectric material itself, but also for compensating, as much as possible, deterioration in ferroelectric characteristics during the production process in the case where the ferroelectric material is used as an element, for example, SiO2 passivation, capacitor processing and the like, which will be described later.
Therefore, in order to form a ferroelectric memory by combining a ferroelectric capacitor using the ferroelectric material with a semiconductor element, such an artifice or the like is necessary that the ferroelectric capacitor and the transistor are formed separately due to the high crystallization temperature for forming the ferroelectric material. Accordingly, high integration of a ferroelectric memory is difficult because of the complication of the production process, restriction in electrode material used in combination, and the like.
In general, a ferroelectric material thin film is used in a ferroelectric memory, and the formation thereof is attained by using a sol-gel process owing to the simplicity and excellent mass-productivity except for step covering property.
In the sol-gel process, a sol-gel raw material solution having a highly volatile component added in an excess amount of about 10% is used in order to improve the crystallinity and to suppress at minimum deviation of the film composition after the film formation.
However, the excess addition of lead and bismuth components sometimes brings about scattering in film composition distribution caused in the ferroelectric thin film finally formed. Furthermore, the compositional deviation in the film accelerates formation of a hetero-phase (such as BIT and a pyrochlore phase and a fluorite phase of SBT, and the like), and makes difficult to obtain the objective ferroelectric single layer.
In the production of a ferroelectric memory, an electrode material that has sufficient durability against the high temperature baking step because of the high backing temperature for the crystallization of the ferroelectric material described in the foregoing.
For example, it has been said that PZT, which is a solid solution of a PbZrO3 antiferroelectric material and a PbTiO3 ferroelectric material, provides a small load on an electrode material. However, it is said that a baking temperature of from 600 to 750° C. is necessary to ensure a residual polarization value that is practically necessary (T. Nakamura, Technical Research Report of Institute of Electronics, Information and Communication Engineers, ED97-208 (1998), p. 25–32), and thus the load on the electrode material or the like is not small. That is, in the case where a PZT thin film is formed on a standard Pt electrode, so-called film fatigue, i.e., precipitous deterioration of the polarization value, occurs due to repeated inversion.
Therefore, it is necessary to use a complicated complex electrode with an oxide series electrode that is excellent in controllability of fatigue of an expensive ferroelectric material, such as Ir, IrO2 and the like, which is difficult to be processed, or an oxide electrode, such as Pt/IrO2 and the like.
On the other hand, SBT (SrBi2Ta2O9: m=2) which is a bismuth lamellar structure ferroelectric material is receiving attention as a material that is free of fatigue upon inversion repeated 1012 times on a Pt electrode and is being earnestly studied for practical application.
However, upon forming SBT into a thin film, coarse grains are aggregated at a low density to obtain only a deteriorated surface morphology (K. Aizawa, et al., Jpn. J. Appl. Phys., vol. 39, p. 1191–1193 (2000)), and high integration (thin film formation) has not yet been realized at the current situation.
SBT is considerably good in P-E hysteresis form but has a low residual polarization value of from 7 to 10 μC/cm2, and thus, when it is tried to be used in a memory of reading a capacitance of a ferroelectric capacitor having been currently commercialized, there is no margin for polarization characteristics, and sufficient characteristics for practical application have not yet been obtained.
Furthermore, SBT is difficult to lower the temperature for crystallization. Specifically, in order to form SBT into a thin film, such methods have been attempted as high temperature baking at 800° C., a long time baking of as much as 5 hours at a relatively low temperature of about 650° C. (Y. Sawada, et la., Technical Research Report of Institute of Electronics, Information and Communication Engineers, ED98-240 (1999), p. 9–14), and two-step baking combining them (S. Hayashi, et al., Technical Research Report of Institute of Electronics, Information and Communication Engineers, ED98-241 (1999), p. 15–19). However, the load on the electrode material due to thermal history is large beyond comparison to PZT, and there is a considerably large problem upon application of the material to practical use.
In recent years, such a method is proposed that the crystallization (baking) temperature is lowered by doping with La, and BIT (Bi4Ti3O12: m=3) is receiving attention as a material used for the method. The material contains a bismuth lamellar structure, has good fatigue characteristics, has a high transition temperature (Tc) of 675° C., and exhibits considerably stable material characteristics at an ordinary temperature, as similar to SBT.
However, the material requires thermal history at 650° C. for 1 hour. Therefore, the load on the electrode material is still large (B. H. Park, B. S. Kang, S. D. Bu, T. W. Noh, J. Lee and W. Jo, Nature, vol. 401, p. 682 (1999)).
The largest problem of the ferroelectric material is that giant particles are liable to be formed (T. Nakamura, “Studies on Ferroelectric Memory having Floating Gate Structure”, Academic Dissertation of Kyoto University (1998), p. 118–140) and it is significantly difficult to form into a thin film, as similar to SBT.
In order to attain high integration and low voltage driving of a ferroelectric thin film element, it is necessary to form the ferroelectric material itself into an extremely thin film.
However, a thin film of 100 nm or less cannot be formed with good reproducibility due to the surface c of the ferroelectric material. Further, even though it can be formed to have a thickness of 100 nm or less, the ferroelectric characteristics are quickly deteriorated (K. Aoki, et al., Technical Research Report of Institute of Electronics, Information and Communication Engineers, ED98-245 (1999), p. 43–49).
It is considered that the surface morphology deterioration of the ferroelectric thin film is caused because the crystallization occurs from a lower electrode (for example, a platinum electrode), i.e., from the lowermost surface of the ferroelectric thin film, irrespective to the film forming method, such as the sol-gel process and the MOCVD process, so as to provide a form having grains convex upward aggregated. Furthermore, although the compatibility between the lower electrode material and the ferroelectric material is poor, the crystallization of the ferroelectric material depends only on the catalytic feature of the platinum electrode, and therefore, the density of initial crystallization nucleus formation of the ferroelectric material is low. Therefore, when the ferroelectric material is formed into a thin film of 100 nm or less, the ferroelectric thin film grows in an island form but not covering the entire lower electrode. As a result, considerably coarse surface morphology is obtained, and the resulting ferroelectric thin film increases the leakage electric current density. It has been known that a ferroelectric thin film derived from an organic metallic material as a starting material has a large amount of carbon remaining, and it is also one factor of increase in leakage electric current density.
Moreover, the ferroelectric material is deteriorated in ferroelectric characteristics in a reducing atmosphere (Y. Shimamoto, et al., Appl. Phys. Lett., vol. 70, p. 1–2 (1997)).
For example, in the case where the ferroelectric material is used as a capacitor, SiO2 passivation using ozone TEOS or the like is generally carried out as a protective film of the ferroelectric capacitor. At this time, the ferroelectric capacitor having a platinum upper electrode formed is exposed to a hydrogen atmosphere. Therefore, hydrogen activated by the catalytic function of the platinum upper electrode reduces the ferroelectric material to cause structural breakage of the ferroelectric material, and as a result, the ferroelectric characteristics are greatly deteriorated.
It is considered that the reasons therefor are that the ferroelectric material is a material having a strong ionic bonding property (while it is said that an ionic bond is a strong bond, it is considerably weak against attack by ions), and the effective area receiving attack by the deteriorated surface morphology is large.
Accordingly, in order to restore the characteristics of the ferroelectric material suffering the structural breakage, re-oxidation of the ferroelectric material is carried out, for example, it is again baked in an oxygen atmosphere, after the SiO2 passivation.
However, the oxidation applies unnecessary thermal history to the element, and moreover, the ferroelectric characteristics once deteriorated cannot be completely restored by the re-oxidation.
Under the current circumstances, accordingly, there are various problems, such as (1) the excess addition of a highly volatile component, such as lead, bismuth and the like, (2) the difficulty in obtaining a ferroelectric single phase, (3) the high crystallizing temperature, (4) the presence of a large amount of carbon residue in the film, (5) the difficulty in forming a thin film of 100 nm, (6) the decomposition under a reducing atmosphere, such as hydrogen or the like, and the like problems, although various kinds of ferroelectric materials have been studies, and thus high integration of ferroelectric thin film elements has not yet been realized.
PZT, a representative perovskite ferroelectric material (ABO3), causes polarization inversion fatigue on a conventional platinum electrode, and SBT and BIT, a bismuth lamellar structure ferroelectric material (BiAm−1BmO3m+3), are difficult to be formed into a thin film due to the deteriorated surface morphology. That is, the ferroelectric materials that are expected to be applied to a memory element under current situation involve respective problems.
The problems are the case not only in the ferroelectric material, but also in a SrRuO3 perovskite electrode material and a perovskite oxide material, such as (Ba, Sr)TiO3, SiTiO3 and the like, which is expected as a high dielectric constant gate oxide film for a next-generation DRAM.